1. Field of the Invention
The present invention relates to autostereoscopic and related images produced by concurrently printing images on paper or plastic and fabricating a lenticular polymer lens surface. The lenticular pattern is produced by applying a reactive polymer layer to the outer surface of a transparent cylinder having a lenticular pattern in relief on its outer surface and casting the pattern in registration with the image using radiation from an ultraviolet light source.
2. Background of the Invention
Autostereoscopic imaging is a technique for providing three-dimensional images. Autostereoscopic imaging is described in U.S. Pat. No. 5,113,213 to Sandor et al. This patent describes a method and apparatus for making autostereographic images by interleaving a number of images in a computer and printing the interleaved images on a printer. The specification, and drawings of this patent are incorporated herein by reference.
Depth Perception
Stereo vision results principally from three depth cues: binocular parallax, monocular movement parallax, and psychological factors. Binocular parallax is due to the spacing between the observer's left and right eyes. Because of the distance between the eyes, light rays from a given object enter the left eye at a slightly different angle than light rays from the same object enter the right eye. The brain integrates the two physical images received by the eyes into one perceived image. The brain deduces the distance to the object from the difference in the angles. The threshold distance for the minimum perception of binocular parallax is approximately 10 inches.
Monocular movement parallax arises from the shift in the image of an object as the angle from which that object is viewed changes. Monocular movement parallax can be perceived even from a single eye, as long as that eye is moving. The more rapidly the eye moves, the more acute the monocular movement depth perception becomes.
Psychological factors such as size, haze, and gradients in shading, shadows and texture also contribute to depth perception. We know the actual size of most objects from experience and memory. As an object recedes in the distance, the size of its image on an observer's retina becomes smaller. Therefore, the size of the image of a familiar object on the observer's retina is inversely related to the distance to the object. In still photography this principle can be employed to trick the viewer into believing that greater than actual depth is being perceived by reducing the size of background objects in the photograph.
The psychology of depth perception dates back to 280 A.D. when Euclid defined binocular parallax as the means by which each eye receives the simultaneous impression of two dissimilar images of the same object. This basic principle is applied today in methods of autostereography using lenticulated lens sheets or barrier strip systems which provide the optics necessary for the perception of depth, without the need for the observer to wear filters or glasses.
Background of 3-D Images
With the invention of photography, it was discovered that two photographs of the same object taken from slightly different viewpoints, if presented to each eye as independent images, would produce a three-dimensional image. Several inventors in the 1800s developed hand held stereoscopes.
To present three-dimensional images to a large audience, a pair of stereoscopic images would be projected in two separate colors and viewed with glasses having complementary color filters over each eye. This anaglyphic method for viewing three-dimensional images with glasses was also used for early three-dimensional motion pictures and for printed images when filtered glasses could be provided. The common colors used for images and filters were red and blue or red and green: the red filter neutralizes the red images and the blue or green filter neutralizes the blue or green image. This approach could not be used for full color images.
When color film became available for motion pictures, the left and right images could no longer be separated using color filters. Polarization filters were developed to replace the color filters. However, polarized filters cannot be used for three-dimensional television, because the television screen cannot display polarized images. The most successful systems developed for three-dimensional television separate the images in time, instead of by polarization. These systems use a liquid crystal shutter on the televised image synchronized with shutters on the left and right eyes. However, this system requires the observer to wear liquid crystal shutters over the eyes synchronized with the left and right eye views of the CRT images.
The color, polarizer and shutter stereoscopic systems described above are not autostereographic systems because the observer must wear an optical device, such as eyeglasses having color filters, polarizers, or electronic shutters to perceive the three-dimensional image.
Autostereography
Autostereography was invented in 1908 by M. G. Lippman. Lippman developed the fly's-eye lens lenticular sheet. The lens sheet contained thousands of small convex lenses arranged either in a random or in an oriented array pattern. A photographic plate placed at the focal plane of the lenses was exposed through a large diameter lens to light reflected from an object. The film recorded the thousands of small photographs as a large integral photograph. A three-dimensional image could be perceived from any angle when a positive of the integral photograph was placed in exactly the same position relative to the convex lens array as the photographic plate. A second kind of autostereogram was developed to make the registration process less demanding. The arrays of convex lenses were replaced with planar arrays of cylindrical lenses. The cylindrical lenses preserved only the horizontal parallax information--the vertical parallax information was lost. Such an image could be viewed from left to right, but not up and down. This approach made registration easier, since only horizontal registration was required.
The lenses in the lenticular sheet were separated from the image by the remaining thickness of the lenticular sheet such that the lenses would focus on the image at the rear surface of the material. A photographic emulsion was placed against the rear surface of the sheet, and an exposure was made through the lenticular sheet. The image plane was moved horizontally during the exposure, resulting in fine columns of left and right eye images. This structure is perceived as a three-dimensional image when viewed through a lenticular screen.
Barrier strip stereograms are a third kind of autostereogram. In barrier strip autostereograms, images are viewed through fine transparent vertical slits in an opaque surface. Parallax factors affect barrier strip three-dimensional images in the same manner as lenticular lens sheets. Barrier strip stereograms must be illuminated from the rear, since ambient room light will not effectively illuminate the image from the front. The much brighter illumination is necessary because only 10-20% of the barrier strip is transparent--the remaining 80-90% is opaque.
Lenticular screen systems thus have an advantage over barrier strip systems, in that images in lenticular systems can be viewed in either the reflection or the transmission modes.
Manufacture of Lenticular Sheets
The manufacture of lenticular sheets requires engraving a master relief pattern from which replications could be made. A number of conventional manufacturing methods have been adapted to produce lenticular sheets with the desired optical characteristics. These manufacturing methods include tooling, platen press, injection or compression molding, embossment, extrusion, and casting. The materials used include a variety of clear optical materials including glass and many types of plastics. Each of these prior art methods suffer inherent problems which render them ineffective for the high-volume production of lenticular screens for autostereography.
Machining can be used to directly manufacture coarse, one-of-a-kind large lenticular screens in thick plastic sheets. Milling machines or lathes can be used with a diamond tip tool having a pre-determined radius. However, machining is a slow and costly process. This method for manufacturing lenticular screens is not well-suited to volume production.
A platen press can be used to stamp or emboss an engraved relief pattern into a thermoset material. The temperature of the thermoset material is raised to soften the material so that it conforms to the engraved surface. The temperature of the material is reduced to harden the material such that it retains the relief pattern when removed from the platen press. Like machining, this method is slow and expensive. Furthermore, the sheet size is limited. This method is not suited for high volume production or for producing a continuous length product. Similar problems apply to injection or compression techniques for manufacturing molded lenticular screens.
The most common method for manufacturing high-volume lenticular sheets is by extrusion embossment in continuous length roll form. Typically, these systems utilize an engraved roller with a thread-like screw pitch to the relief pattern. The quality and definition of extrusion relief patterns are generally inferior to patterns obtainable by platen or ultra-violet casting methods.
Extrusion techniques have difficulty maintaining the absolute parallelism of the lenticular rows. Because of the elastic nature of the molten plastic material and the internal stresses imparted by the embossing roller, the sheet has a tendency to change from its impressed shape prior to being fully set. Additionally, extrusion lenticular sheets can streak due to condensation, adding to the dimensional distortion and migration of the lenticular surface. These physical distortions optical defects in the lenticular sheet result in serious distortions and degradations in the perceived image. Migration is the tendency of the extruded plastic to move in a direction perpendicular to the direction of lenticulation during the extrusion process.
The optical quality of extruded lenticular sheets also suffers from the influences of the polymers from which they are formed. Some extrusion systems attempt to control this problem by curtain coating the polymers to a pre-extruded non-lenticulated web sheet requiring a binder coating to produce the multi-layered ply-sheet. Curtain coating is a process in which a flow of liquid plastic is set by a chill roller. This does not control the migration problem and adds defects such as bubbles, separation of surfaces, and diffusion of images, thus reducing the optical quality of the lenticular sheet.
These problems were addressed by a photographic technique using a thermoset UV casted sheet developed in the 1980s. The technique used a composite sheet having a back surface coated with a photosensitive emulsion. The stereoscopic images were obtained as multiple exposures of the photosensitive emulsion through a lenticular screen. The composite sheet had a layer of cured thermosetting polymer on one surface of a base polymer film. The patterned lenticular relief was imposed upon the thermoset layer by curing the thermosetting resin while it is wrapped around a molding surface. The technique requires that it be used only with continuous roll transparent films. The disadvantage of this approach is that only special dedicated equipment could produce overall full-width lenticulated continuous roll transparent films.
The needs for printing applications are very different. Lenticulation is only required in the three-dimensional picture area of a sheet or page of a book, and not on the entire surface. When the lenticulation covers an area of text, the text becomes much harder to read. Additionally, fully lenticulated pages add an unnecessary cost to the finished product.
Geometry of the Lenticular Sheet
The geometry of the lenticular sheet is determined by the conditions under which the image is obtained. To create autostereographic images, the object is photographed at three or more slightly different viewing angles, simulating the parallax created by the average 6.5 cm separation of two eyes. The difference in the viewing angles is determined by the distance from which the autostereoscopic display is to be viewed (the "viewing distance").
As the viewing distance increases, the pitch of the lenticular sheet becomes increasingly coarse and the thickness of the sheet increases in order to retain focus as the lens radius increases with pitch. The pitch is the number of lines per inch of the image. The format size of the image also generally increases with the viewing distance. Small hand-held lenticular sheet autostereographs require a very fine pitch, e.g., from 80 to 300 lines per inch. Larger lenticular screen autostereographs can have a pitch with as few as 10 to 40 lines per inch. As the cylindrical lenticules become smaller, in finer pitch lenticular screens, the thickness of the lenticular sheet is also reduced so that the lenticules can focus upon the back surface of the screen.
To produce high quality autostereographs, the lenticular sheet itself must have good depth resolution. Assume that the thickness of the lenticular sheet selected will allow a bundle of parallel incident beams of light to focus on the back surface of the sheet. The light beams thus form a single spot so that an observer cannot distinguish more than one spot inside any cylindrical lenticule from a given position. The width of the lenticule determines the minimum resolvable lateral picture element.
When lenticular screens are used with pre-established image pitches, such as in photo duplication or in printed images, the lenticules must be consistently and reliably identical and parallel, in order to register to the pre-established image. Lens aberrations, or variations in the thickness of the spacer sheet, may misdirect the outgoing beams, so that the perceived image is other than the defined image at a given viewing angle. However, when the lenticular screen is used with a photosensitive emulsion applied to the back side of the lens, and projections of the stereo images are made through the lens, most of the distortion due to migration or non-parallelism is self-cancelling.
Except for extreme variations, these distortions do not effect the photo-composed (one-of-a-kind) three-dimensional images. However, dedicated three-dimensional photography, photo-mechanical combinations or graphic arts separation and scanner-generated lenticular patterns require a standard pitch. The display lenticular screens must be made to conform exactly to the pitch and parallelism of the lenticular system used to generate the original image. Correction must also be made for the parallax angle when determining the pitch of the lenticular screen.